Not Applicable
Not Applicable
The present invention relates to methods for attaching closure members, i.e., a zipper tape, to a plastic film web. It appears especially well suited for laminating zipper elements to plastic storage bags so as to form reclosable storage bags.
The prior art has developed reclosable plastic storage bags for use in protecting food items and for other storage purposes. A film of a plastic such as polyethylene is provided with a closure member (one half of a zipper) on its opposed ends. The film is then folded to bring the zipper halves adjacent to each other, with the sides of the bag then being heat sealed. The bag can then be opened or closed in the usual manner by interlocking the zipper elements.
One difficulty experienced by manufacturers of reclosable plastic storage bags involves the joining of the closure members to the bag film web portion. Care must be taken to ensure that the closure members are positioned properly on the bag film web. Further, there should be no wrinkles or creases in the area of the bag in which the fastener profiles and bag film web are joined.
A variety of methods are known for attaching closure members to a plastic film web prior to forming reclosable plastic storage bags. One approach has been to form the bag film web and the closure members in a single extrusion operation. See e.g. U.S. Pat. Nos. 3,338,284 and 3,852,386. The disclosure of these publications and of all other publications referred to herein are incorporated by reference as if fully set forth herein.
A second approach utilizes an adhesive, such as a strip of tape or glue, to join a closure member to a bag film web. For example, U.S. Pat. No. 4,101,355 discloses a reclosable bag film forming process wherein a web and closure member are joined by a liquid adhesive which is supplied by an applicator having a nozzle. After the adhesive is applied, the web and closure members are pressed together by a presser roll and a backing roll. See also U.S. Pat. Nos. 4,341,575 and 3,226,787.
A third approach involves the use of a pre-formed bag film web and a pre-formed closure member which are drawn from separate stock rolls. The bag film web and/or closure member are then heated and joined. For example, U.S. Pat. No. 3,846,209 discloses a method for producing a closure member-containing bag film wherein a pre-formed plastic film is fed from a stock roll into a sealing station. A separate closure strip from a second stock roll is also fed into the sealing station. In the sealing station, the closure strip and plastic film are welded together by a pair of heating bars.
A fourth approach uses a pre-formed bag film web drawn from a supply roll and then joined to a relatively freshly extruded closure member. For example, U.S. Pat. No. 3,904,468 discloses a method of making a reclosable bag film wherein a web of bag film material is unwound from a stock roll and then transferred to a heating station having a heating roll on which the film is heated to a desired temperature. The film is then transferred to a heated joining roll wherein the film is joined to the freshly extruded fastener profile strips. See also U.S. Pat. Nos. 4,279,677 and 4,582,549.
Another variation on this fourth approach is disclosed in U.S. Pat. No. 4,555,282 which discloses a method of bonding a closure member to a bag film web wherein a closure member having a base portion and a fastener profile portion is extruded shortly before being bonded to a bag film web. Intermediate the extrusion of the closure member and its joinder to the bag film web, the fastener profile portion of the closure member is cooled (and thereby solidified and stabilized) while the base portion is left heated (and thereby remains thermoplastic) when the closure member is joined to the film.
U.S. Pat. No. 4,306,924 discloses another variation on this fourth approach wherein the film web is extruded on to a casting roll to join an already formed closure member.
A fifth approach involves the.extrusion of a bag film web and a closure member in close proximity to each other, and the joinder of the bag film web and closure member to each other shortly after extrusion. For example, U.S. Pat. No. 4,428,788 discloses a method for forming a reclosable bag wherein a film, a tape, and a closure member are extruded by three separate dies. The film, tape and closure member dies are positioned so that the film, tape and closure members contact each other while still at a temperature of above 200xc2x0 F. A chill roll is provided for cooling the film, tape and closure members.
U.S. Pat. No. 4,295,915 discloses an apparatus for forming a reclosable bag film wherein a fastener profile and its base strip are extruded integrally, and then joined to a film web. The die for extruding the bag film web is disposed adjacent to the fastener die block so that the fastener profile and bag film are joined shortly after both are extruded. See also U.S. Pat. Nos. 3,462,332 and 4,522,678.
A sixth approach involves the extrusion of a bag film web having a relatively thinner first portion and a relatively thicker second portion, and the separate extrusion of male and female fastener profiles that are joined to the thicker second portions before the thicker second portions have cooled. An example of this approach is found in U.S. Pat. No. 4,755,248.
A seventh approach involves attaching a closure member for a storage bag to the bag via an intermediate heat setting adhesive. See eg. U.S. Pat. Nos. 5,749,658 and 5,827,163.
However, to keep the costs of producing such bags to the minimum it is desirable to use high speed equipment to form these bags. Prior methods may not achieve a consistent seal between the closure members and the film web (and may introduce tension differences across the film web) when run at a very high speed. In addition, room for improvement exists in the fabrication of a reclosable bag which can be manufactured to consistently high quality standards.
There is therefore still a need for improved methods of attaching closure elements to plastic film such as that used in forming reclosable storage bags.
The invention provides methods for forming a thermoplastic film having a closure strip. In one method, one provides a thermoplastic film web having a sealing surface, the temperature of the film web being below a melting temperature of the film web. One also provides a thermoplastic closure strip having a binding surface, the temperature of the closure strip being below a melting temperature of the closure strip.
One extrudes a thermoplastic binder layer, and then positions the binder layer between the sealing surface of the film web and the binding surface of the closure strip. The binder layer contacts the sealing surface of the film web before the binder layer has cooled to a temperature below the melting temperature of the film web and the binder layer also contacts the binding surface of the closure strip before the binder layer has cooled to a temperature below the melting temperature of the closure strip. Pressure applied to the film web and the closure strip binds the closure strip and film web to the binder layer.
In preferred forms there can be multiple closure strips (such as interlockable male and female strips), multiple binder layers (one binding each of the strips to the web), and the invention can be practiced on a folded web at opposite ends of the web.
In another aspect there is a first cylindrical roller having a surface and an axis of rotation and a second cylindrical roller having a surface and an axis of rotation, the axis of the first roller and the axis of the second roller being substantially coplanar, the first roller and the second roller being aligned in spaced apart relationship such that a pinch area is formed between the surface of the first roller and the surface of the second roller.
One feeds a continuous length of a thermoplastic film web having a sealing surface, such that the film web wraps on the surface of the first roller, while the temperature of the film web is below a melting temperature of the film web; and feeds a continuous length of a thermoplastic closure strip having a binding surface such that the closure strip wraps on the surface of the second roller, while the temperature of the closure strip is below a melting temperature of the closure strip.
One extrudes a thermoplastic binder layer, positions the binder layer between the sealing surface of the film web and the binding surface of the closure strip, and feeds the film web, binder layer and closure strip through the pinch area such that the binder layer contacts the sealing surface of the film web before the binder layer has cooled to a temperature below the melting temperature of the film web. The binder layer contacts the binding surface of the closure strip before the binder layer has cooled to a temperature below the melting temperature of the closure strip.
In another method one forms a folded thermoplastic film having a female closure strip and a male closure strip at opposed ends of the film. One step is to provide a first cylindrical roller having a surface and an axis of rotation and a second cylindrical roller having a surface and an axis of rotation, the axis of the first roller and the axis of the second roller being substantially coplanar, the first roller and the second roller being aligned in spaced apart relationship such that a first pinch area is formed between the surface of the first roller and the surface of the second roller.
Another step is to provide a third cylindrical roller having a surface and an axis of rotation and a fourth cylindrical roller having a surface and an axis of rotation, the axis of the third roller and the axis of the fourth roller being substantially coplanar, the third roller and the fourth roller being aligned in spaced apart relationship such that a second pinch area is formed between the surface of the third roller and the surface of the fourth roller.
One then feeds a continuous length of a folded thermoplastic film web having a first sealing surface on a first edge spaced apart from a fold line of the film web and a second sealing surface on a second edge spaced apart from the fold line of the film web such that the first edge of the film web wraps on the surface of the first roller and the second edge of the film web wraps on the surface of the third roller, while the temperature of the film web is below a melting temperature of the film web. One also feeds a continuous length of a thermoplastic female closure strip having a binding surface, such that the female closure strip wraps on the surface of the second roller, while the temperature of the female closure strip is below a melting temperature of the female closure strip.
Another step is to extrude a first thermoplastic binder layer and then position the first binder layer between the first sealing surface of the film web and the binding surface of the female closure strip. One then feeds the film web and the female closure strip through the first pinch area such that the first binder layer contacts the first sealing surface of the film web before the first binder layer has cooled to a temperature below the melting temperature of the film web and the first binder layer contacts the binding surface of the female closure strip before the first binder layer has cooled to a temperature below the melting temperature of the female closure strip.
One also feeds a continuous length of a thermoplastic male closure strip having a binding surface, such that the male closure strip wraps on the surface of the fourth roller, while the temperature of the male closure strip is below a melting temperature of the male closure strip.
Another step is to extrude a second thermoplastic binder layer and position the second binder layer between the second sealing surface of the film web and the binding surface of the male closure strip. One then feeds the film web and the male closure strip through the second pinch area such that the second binder layer contacts the second sealing surface of the film web before the second binder layer has cooled to a temperature below the melting temperature of the film web and the second binder layer contacts the binding surface of the male closure strip before the second binder layer has cooled to a temperature below the melting temperature of the male closure strip.
In another aspect the invention provides an apparatus for forming a thermoplastic film having a closure strip. There is a cylindrical lamination roller having a surface and an axis of rotation. There is also a cylindrical lay-on roller having a surface and an axis of rotation. The axis of the lamination roller and the axis of the lay-on roller are substantially coplanar, and the lamination roller and the lay-on roller are aligned in spaced apart relationship such that a pinch area is formed between the surface of the lamination roller and the surface of the lay-on roller.
There is also a web feed roller for feeding a continuous length of a thermoplastic film web having a sealing surface, the web feed roller being positioned such that the film web can wrap on the surface of the lamination roller after leaving the web feed roller. Another element is a closure strip feed roller for feeding a continuous length of a thermoplastic closure strip having a binding surface, the closure strip feed roller being positioned such that a closure strip can wrap on the surface of the lay-on roller.
An extruder can extrude a heated thermoplastic binder layer, the extruder including a die block with an opening, and the extruder being positioned such that the opening in the die block is adjacent the pinch area so as to be suitable to direct a heated binder layer between a nonheated closure strip and a nonheated film web.
The above methods and equipment permit the production of film webs with closure members assembled thereon at very high speeds (e.g. about 200 linear feet per minute). When the methods are used to form bags, the seal between the bag and zipper will be highly reliable and consistently formed (thus permitting an airtight structure).
Particularly preferred thermoplastics for the film web are low or high density polyethylene, linear low density polyethylene, polypropylene, nylon, and interpolymers of ethylene and one or more monomers copolymerizable therewith.
It is particularly preferred to have the closure members formed from the same thermoplastic as the film web. However, this is not essential. It is also preferred that the binder layer be formed from the same thermoplastic as the film web. However, this is also not essential.
It is particularly important that the binder layer thickness be sufficiently great so as to carry enough heat to create a melted region adjacent the binder layer when the binder layer contacts the closure members and the film web. At the same time, the binder layer should be sufficiently thin so as to create a stable cooled structure in under a second after the binder layer is pressed between a closure member and the film web.
It is an advantage of the present invention to provide a rapid method for attaching prefabricated thermoplastic closure members to prefabricated thermoplastic film webs traveling at high speeds.
This and still other advantages of the present invention will be apparent from the description of the preferred embodiments which follow.